John IV may refer to:

Blessed Pope John Paul II (Latin: Ioannes Paulus PP. II, Italian: Giovanni Paolo II, Polish: Jan Paweł II), born Karol Józef Wojtyła (Polish: [ˈkarɔl ˈjuzɛf vɔjˈtɨwa]; 18 May 1920, Wadowice, Republic of Poland – 2 April 2005, Apostolic Palace, Vatican City), reigned as Pope of the Catholic Church from 1978 until his death in 2005. He was the second-longest serving Pope in history and the first non-Italian since 1523.

John Paul II was acclaimed as one of the most influential leaders of the 20th century. He was instrumental in ending communism in his native Poland and eventually all of Europe. John Paul II significantly improved the Catholic Church's relations with Judaism, Islam, the Eastern Orthodox Church, and the Anglican Communion. Though criticised by progressives for upholding the Church's teachings against artificial contraception and the ordination of women, and by traditionalists for his support of the Church's Second Vatican Council and its reform, he was also widely praised for his firm, orthodox Catholic stances.

John Calvin (French: Jean Calvin, born Jehan Cauvin: 10 July 1509 – 27 May 1564) was an influential French theologian and pastor during the Protestant Reformation. He was a principal figure in the development of the system of Christian theology later called Calvinism. Originally trained as a humanist lawyer, he broke from the Roman Catholic Church around 1530. After religious tensions provoked a violent uprising against Protestants in France, Calvin fled to Basel, Switzerland, where he published the first edition of his seminal work The Institutes of the Christian Religion in 1536.

In that year, Calvin was recruited by William Farel to help reform the church in Geneva. The city council resisted the implementation of Calvin and Farel's ideas, and both men were expelled. At the invitation of Martin Bucer, Calvin proceeded to Strasbourg, where he became the minister of a church of French refugees. He continued to support the reform movement in Geneva, and was eventually invited back to lead its church.